During the last two decades a variety of organosilicon compounds have been shown to react with various electrophilic reagents. Reactions may occur with organosilicon compounds containing multiple bonds which are one, two or three atoms removed from silicon, i.e., with arylsilanes, vinylsilanes, alkynylsilanes, silyl enol ethers, allylsilanes, benzylsilanes, homoallylsilanes and under vigorous conditions also with alkylsilanes. Most of these reactions are envisioned to proceed by electrophilic attack leading to an intermediate cation beta to silicon. Such reactions are highly regioselective due to cation stabilization. The silyl group is usually lost during subsequent steps leading to compounds having the electrophile and the multiple bond in predictable locations.
Electrophilic substitution of organosilicon compounds is one of the least explored synthetic techniques in polymer synthesis. Due to the relatively weakly polarized silicon-carbon bond organosilanes behave as weakly reactive organometallic compounds. Thus they can be handled more conveniently than other organometals, i.e., they do not usually require anhydrous or inert atmospheres and are inert in the presence of a great variety of functional groups. Little work has been done on electrophilic substitution of organosilicon compounds with carbocations or species bearing a relatively high positive charge on the carbon atom. Adamantyl and tert-butyl halides have been demonstrated to undergo substitution in the presence of Lewis acids with select unsaturated organosilicon compounds. (See for example, I. Fleming, et al. Synthesis, 1979, 446; T. Sasaki, et al., J. Org. Chem., 1980(45), 3559.)
Polyisobutylene has limited utility because it is hard to crosslink. Copolymerization with small amounts of isoprene was found to give residual sites of unsaturation which thus permitted sulfur vulcanization, resulting in the commercialization of butyl rubber during World War II. Besides chemical and ozone inertness, butyl rubber has very low permeability to gases and has thus found widespread use in tire inner tubes. Low molecular weight polyisobutylene oils are currently used to increase the viscosity of lubricating oils, and the higher molecular weight unvulcanized polymer is used in adhesives, caulks, sealants, and polymer additives.
Copolymerization of polyisobutylene with polydialkylsiloxanes, so-called silicones, would produce desirable materials. Surprisingly, very little work has been done with soft block-soft block copolymers of polydimethylsiloxane (PDMS) with either polyisobutylene or other organic polymers that are above their glass transition and crystal melting temperatures at ambient temperature. Such copolymers are expected to be fluid materials. PDMS polybutadiene soft block-soft block copolymers of comb structure are known but do not have the ozone and yellowing resistance that a PIB silicone block copolymer would have.
A simple way to join a polydimethylsiloxane polymer to an organic polymer to form a block copolymer is through the hydrosilylation reaction which involves the platinum catalyzed addition of an SiH moiety to most preferably a terminal olefin, H.sub.2 C.dbd.CHR, to give SiCH.sub.2 CH.sub.2 R.
Industrially, isobutylene is polymerized with aluminum chloride at reaction temperatures as low as -100 degrees Centigrade. The product has mostly saturated aliphatic end groups.
Polyisobutylene (PIB) containing sites of unsaturation can be produced by copolymerization of isobutylene with small amounts of isoprene. The resulting unsaturation permits vulcanization, but because the sites are mainly internal, hydrosilylation is inhibited or prevented. Terminal olefinic end groups on one end can be obtained by initiating polymerization with BCl.sub.3, and CH.sub.2 .dbd.CHC(CH.sub.3).sub.2 Cl, but not with allyl chloride. The other end of the macromolecule will be chloride ended. Although the CH.sub.2 .dbd.CHC(CH.sub.3).sub.2 -- group is terminal, it is still not very reactive in hydrosilylation due to the steric hinderance provided by the two methyl groups. In addition, only an (AB) block copolymer may form, where A represents the siloxane block and B represents the hydrocarbon block. Another approach has been to make a polymer with chlorine at each end by using a special dichlorocarbon coinitiator such as para dicumyl chloride with BCl.sub.3, or by using chlorine as a coinitiator. The chlorine terminated polymer is then dehydrohalogenated to form the --CH.sub.2 --C(CH.sub.3).dbd.CH.sub.2 group by refluxing 20 hours with potassium tertiary butoxide, cooling, water washing three times, and drying. (See U.S. Pat. No. 4,342,849, issued Aug. 3, 1982 to Kennedy). This terminally unsaturated PIB can slowly undergo hydrosilylation. Hydrosilylation is slow because the end group is sterically hindered. Thus there exists a need for a fast, simple and inexpensive method to provide unhindered allylic, CH.sub.2 .dbd.CHCH.sub.2 --, terminal functionality on PIB to produce polymers such as CH.sub.2 .dbd.CHCH.sub.2 --PIB--CH.sub.2 CH.dbd.CH.sub.2 which can undergo rapid hydrosilylation at both ends to form an (AB).sub.x block copolymer where x is greater than two. Such materials are useful in many applications, including use as electronic potting gels, surfactants to compatibilize PIB with silicones, pressure sensitive adhesives and as non-stick chewing gum.